1. Field of Invention
This invention relates to a printed wiring board provided with via-holes formed by using a laser beam and a method of manufacturing the same.
2. Description of Related Art
A printed wiring board, as shown for example in FIG. 61, is formed by laminating plural insulating layers 91˜93 one upon the other and electrically connecting innerlayer conductor circuits 95, 97 arranged between the layers to an outer conductor circuit 94 existing on an outermost surface through metal plated films 96 formed on via-holes 99a, 99b. A resin substrate such as glass-epoxy substrate or the like is used to form insulating layers 91-93. The via-holes 99a, 99b are blind via-holes each covered at one end and opened at the other end.
There are various methods of forming via-holes, such as using a drill, using a laser beam, etc.
However, these drilling methods cause unevenness in the depth accuracy. And also, it is recently demanded to make more fine a diameter of the via-hole with the advance of high densification in a wiring pattern.
For this end, there is proposed the formation of holes through the laser beam JP A62 216297, JP A 3 165594.
According to the latter method, as shown in FIG. 62, the via-holes 99a, 99b can be formed in a region ranging from an outermost surface of a multi-layer laminated plate up to innerlayer conductor circuits existing therein by utilizing a property that laser beams 81 reflect on the innerlayer conductor circuits 95, 97, each made of a metal such as copper foil, metal plated film or the like.
In the printed wiring board, however, a relatively shallow via-hole 99a arranged at the innerlayer conductor circuit 95 near to the outermost surface of the printed wiring board and a relatively deep via-hole 99b arranged at the innerlayer conductor circuit 97 far away from the outermost surface are existent together as the via-hole as shown in FIG. 62.
As a result, when the holes are formed through the laser beam 81, as shown in FIG. 63, there may be damage 98 such as breakage, swelling and the like, in a central portion of the innerlayer conductor circuit 95 arranged in the bottom of the shallow via-hole 99a although the innerlayer conductor circuit 97 arranged in the bottom of the deep via-hole 99b is not damaged.
Once such damages are caused, when a metal plated film is formed on each surface of the innerlayer conductor circuit 95 and via-hole 99a and between such a surface and an outer conductor circuit, pieces 98a of the innerlayer conductor circuit broken due to the damage 98 remain in the via-hole 99a, so that the metal plated film may not be formed sufficiently. Such an insufficient metal plated film badly affects electric conduction between the innerlayer conductor circuit 95 and the outer conductor circuit 94 and sometimes rejects the printed wiring board itself.
The reason why the damage is caused in the innerlayer conductor circuit 95 as mentioned above is due to the fact that laser beams 81 having an intensity required for the formation of the deep via-hole 99b are irradiated to not only the shallow via-hole 99a but also the deep via-hole 99b as shown in FIG. 62.
Therefore, it is also considered to adjust a laser beam intensity in accordance with the depth of the via-hole to be formed. However, the number of via-holes formed in one printed wiring board is as many as, for example, 500˜10000. Therefore, it is complicated to adjust the intensity of the laser beam for every via-hole and also requires an expensive control device for such an adjustment.
Furthermore, a black oxide film is formed on the surfaces of the innerlayer conductor circuits 95, 97 for ensuring an adhesion property to the insulating layers 91, 92. Since the black oxide film is high in the energy absorption of laser beam, there is a fear of causing damages such as breakage and the like due to fusion and swelling of the innerlayer conductor circuit provided on its surface with the black oxide film.
Also, the insulating layer made of the resin or the like is exposed in the via-hole, so that it is difficult to form the metal plated film. For this end, the joining property between the inner wall of the via-hole and the metal plated film is low resulting in peeling, dropping-out and the like of the metal plated film. This can have a bad effect on the electric conduction of the via-hole.
The formation of the metal plated film on the via-hole is carried out by flowing an electric plating solution into the via-hole. However, as the inner diameter of the hole becomes small, it is difficult to make the formation of the plated film uniform, resulting in the electric conduction of the via-hole being insufficient.
Further, it has been attempted to provide the electric conduction by using solder to fill in the via-hole instead of the metal plated film. In this case, the joining strength between the inner wall of the via-hole and the solder is low. Also, the solder may be forced out from the via-hole through external pressure.
Moreover, it is required to utilize the irradiation of the laser beam to form holes other than the via-hole.
In the light of the above problems, the invention provides a printed wiring board and a method of manufacturing the same which can form via-holes having reliable electrical conduction without causing damage of innerlayer conductor circuits through a laser beam and furthermore to realize novel circuit formation through the laser beam.